A. Field of the Invention
The present invention relates to equipment used in electrical power distribution systems which utilize pole-mounted power lines and distribution transformers. More particularly, the invention relates to a novel seismic-resistant cluster bracket for mounting to a power pole and supporting multiple distribution transformers, which utilizes adjustable-span diagonal braces to limit damaging and potentially catastrophic displacement of the cluster bracket and transformers from the pole during a seismic event.
B. Description of Background Art
Systems for distributing electrical power produced by power generation stations which uses hydro, nuclear, or fossil-fuel energy sources typically employ a cascaded arrangement of transmission lines. A typical distribution network includes high tension lines carrying current at very high voltages of the order of 110 kV to 220 kV (kilovolts) which are supported by steel towers. The high tension lines carry electrical power from a power generating station to one or more substations, where large, ground mounted transformers step down current in a 110 kV-220 kV voltage range to voltages in an intermediate voltage range of about 2.4 kV to 34.5 kV.
Intermediate voltage range power lines which convey power to locations of businesses, residences, and other consumers of electrical power, are in many cases supported by upright wooden power poles. Although heavy industrial consumers sometimes require higher voltages, three-phase electrical current is usually supplied at a maximum voltage of about 440 volts to industrial users. Typically a three-wire configuration is used to supply both 240-volt and 120-volt current to residential consumers. In both cases, pole-mounted distribution transformers are required to step down current carried by of substation power lines in an intermediate voltage range to lower voltages of 440, 240, or 120 volts.
Since typical pole-mounted transformers provide electrical power to multiple residential and/or commercial electrical power consumers, pole-mounted distribution transformers must handle relatively large power levels, typically of the order of 150-200 kVA or more. Consequently, pole-mounted distribution transformers are large and heavy, often weighing 2,000 lbs. or more. Moreover, many, if not most, electrical power distribution systems utilize three-phase electrical power lines. Although three-phase power transformers are sometimes used to step down power-line current to lower voltages, most distribution systems use three separate electrically interconnected single-phase transformers to step down three-phase substation power. For that reason, power pole installations used to step down power supplied in an intermediate kilovolt range of several kilovolts to 440 v or 220 v power often employ three separate single-phase transformers. The three transformers are usually mounted on a multi-transformer, “cluster bracket” which is bolted to a wooden pole. A cluster bracket is usually fastened to a pole near its upper end, below upper cross arms which support high tension input power lines, and above lower cross arms which support lower-voltage output power lines.
Since, as stated above, each of the three single-phase distribution transformers supported by a cluster bracket can weigh 2,000 lbs. or more, it can be readily appreciated that cluster brackets must have a very rugged construction to enable them to support such a large static weight load.
Currently-used cluster brackets are typically mounted to a pole by two or more bolts which are pre-installed in a power pole at a selected mounting height near the upper end of the pole. The shanks of the bolts protrude radially outwards from the poles, and are received through perforations through rear mounting plates of a cluster bracket. The cluster bracket is then secured in place by tightening nuts on the shanks of the bolts which protrude through the perforations in the mounting plates.
The present inventor has observed that after periods as short as 6 months, static weight loading of heavy transformers supported by a cluster bracket can cause stretching and downward bending of mounting bolt shanks. Also, it is known that lateral and vertical forces exerted on mounting bolts by seismic events can exert even larger dynamic bending and stretching strains on the mounting bolts.
For the foregoing reasons, there is a substantial concern that the cumulative effects of static and dynamic strains on the mounting bolts of currently used cluster brackets may result in the catastrophic failure of such cluster brackets, in the event of seismic events of sufficiently large magnitudes. That concern was a motivating factor in the present inventor's conception and development of a novel improved multi-transformer cluster bracket which provides greater resistance to both static and dynamic deformations of cluster bracket members and mounting bolts than present-generation cluster brackets, thus minimizing the possibility of catastrophic failure of the improved cluster brackets.